1. Field of the Invention
The present invention relates to an optical recording medium capable of rapidly recording/reproducing optical information of high density by utilizing a laser beam.
2. Description of the Related Art
An optical recording medium for recording/ reproducing information of high density by utilizing a laser beam comprises a transparent substrate and a recording layer formed on the substrate. In the conventional optical recording medium, the recording layer is selectively irradiated with a laser beam, with the result that, for example, a pit is formed in the irradiated portion by the heat energy of the laser beam so as to perform the information recording. The mechanism of the pit formation in the recording layer has not yet been clarified in detail. However, two phenomena are considered to accompany the pit-forming mechanism, i.e., (1) evaporation of a material forming the recording layer, and (2) melting of the material forming the recording layer, leading to aggregation of the molten material around the laser beam-irradiated region by the surface tension of the melt. These phenomena cause a geometrical change of the atoms forming the recording layer. In this recording system, the recording layer is formed of a material having a relatively low evaporation temperature and a relatively low melting point. In addition, the material should be low in its heat conductivity in order to improve the resolution. A material consisting of oxides of Bi, Te and In, metal Te and an organic compound is used in general for forming the recording layer included in the optical recording medium of the ablation type described above.
In order to miniaturize and simplify the recording and reproducing apparatuses of the optical recording medium, the recording layer should desirably be capable of information writing by a low power laser such as a GaAlAs semiconductor laser. In particular, a recording layer capable of information writing by a low power laser is of high importance when it comes to an optical recording medium such as an optical card. It should be noted in this connection that the ablation type optical recording medium, which certainly permits information writing in the case of using a high power laser such as an argon gas laser, carbon dioxide gas laser or a YAG laser, is incapable of exhibiting a sufficiently high recording sensitivity in the case of using a low power semiconductor laser. This made it difficult to miniaturize and simplify the conventional recording and reproducing apparatuses.
An optical recording medium such as an optical card may be handled easily, if it is possible to paste a protective substrate with an adhesive directly to the recording layer formed on the transparent substrate. In the ablation type optical recording medium, however, the material forming the recording layer is evaporated, making it unavoidable to employ the so-called "air sandwich structure". It follows that the ablation type optical recording medium leaves room for further improvement in its handling.
Recently, a new type of optical recording medium has been announced. The medium comprises a recording layer formed of a material which brings about phase transition upon laser beam irradiation. However, the recording layer of the phase transition type medium is formed of a complex compound, leading to a low productivity and a high manufacturing cost.
What should also be noted is that, in the optical recording medium, it is necessary to use a pregrooved substrate formed of, for example, a polycarbonate resin because the tracking is performed by detecting the difference in the intensity of the reflected light derived from the difference in the optical paths of the incident light beams. The pregroove providing the tracking line must be precisely formed in a depth of 100 nm or less. In general, the substrate having such a pregroove is formed by injection molding of resin using a mold having a fine pattern of projection corresponding to the tracking line. However, it is necessary to control very strictly the conditions such as the mold temperature in order to manufacture a substrate having a fine pregroove by injection molding.